This invention relates to the field of graphic arts, and in particular to a method and system that compensates for coverage-area spread distortions that occur during printing.
Image Reproduction
The field of graphic arts deals with the reproduction, for example the printing of images, and with the preparation of images for such reproduction. In recent years, the process of preparing data for printing has been computerized, so that image data is typically prepared on a computer by combining the image elements, which might include scanned or digitally acquired photographs and digitally designed graphic elements. Image data thus may include linework and continuous tone images, and therefore may include areas that have various tones. The color at any point in an image is represented by a set of color components, may be the quantities of inks or toners that will be used for printing (e.g., cyan, magenta, yellow, and black) or in some other component system which eventually will need to be converted (xe2x80x9cseparatedxe2x80x9d) into the ink values for printing. Such other components include red, green and blue components (RGB) and other color co-ordinate schemes (e.g., CIE-Lab). Thus, color images are represented by a set of monochrome component images, each representing the amount of one of the color component. When these components are the quantities of inks or toners, the monochrome component images are called separations.
Most printing processes (e.g., offset press, gravure, flexography) are usually capable of printing only two tones of any ink or toner, i.e., either deposit or not deposit ink or toner onto a substrate or carrier, which is usually but not necessarily a sheet of paper. Halftoning is used to reproduce images having continuous tones. Halftoning converts varying values of tints and tones into a geometric distribution of small objects (e.g., dots) that can be printed. The human eye xe2x80x9cspatially integratesxe2x80x9d these small objects over a larger area so that one perceives continuous tones when viewing the image from a distance. The most common form of halftoning uses small dots, with either the size of the dots or the frequency of dots per unit area varying to provide for different grey tones. Halftoning using dots is called traditional halftoning herein.
The steps in the reproduction of an image may include recording the separations of the image on film using a recorder such as an imagesetter, then making a set of plates from the film. Alternatively, a plate may be directly exposed. Other forms of reproduction include flexography, gravure, and direct imaging onto the substrate using electro-photography, for example, xerography. In all these cases, the image is usually converted to picture elements (pixels) which are used to modulate a light source (e.g., a laser) to expose some photosensitive medium. The pixels are usually organized in a raster.
The image data (xe2x80x9cartworkxe2x80x9d) may be available directly in pixel form, or may be provided in some other format, for example, as a page description language (PDL) file such as a PostScript(copyright) file (Adobe Systems Incorporated, San Jose, Calif.), or in some other object-oriented format. Any of these non-pixel formats needs to be converted into a raster stream of pixels prior to exposure, and this conversion process is called raster image processing (RIPping). The device for performing RIPping is called a raster image processor (RIP), and typically comprises a computer running RIP software. Halftoning may be carried out during RIPing, prior to RIPping, or even after RIPing.
Some forms of printing require high accuracy. One example is the printing of security documents such as banknotes, travelers checks, and share certificates, in which the artwork is extremely precise.
Coverage Area Spread Distortions
There typically are deviations in the appearance of a print of reproduced artwork from what is expected. One of these deviations is due to a spread in the area of ink coverage of solid objects. This distortion occurs as a result of making a printing plate (if used) and as a result of the printing process itself. It is called xe2x80x9ccoverage-area gain,xe2x80x9d xe2x80x9ccoverage area spread,xe2x80x9d or simply xe2x80x9ccoverage-gainxe2x80x9d herein, and is present in most printing processes, including lithography, flexography, gravure printing, and even in electro-photography (xerography).
Explanations for the effect include the spread that occurs when an amount of ink is applied to a reasonably large area and then pressed onto the substrate or carrier during printing. The thickness of the ink is then forced over the edges of the area over a small distance. Depending on the printing process, other elements such as ink transfer efficiency, ink absorption in the paper, and optical characteristics of the ink/paper combination can also influence the amount of coverage gain. Sharp inward or outward image edges will tend to fill in or to disappear, so the shape of the ink coverage area is another factor.
The effect of coverage gain is to reduce accuracy of printing, and this is especially noticeable when printing small elements. For example, this is commonly noticeable in halftone printing, where the effect is called xe2x80x9cdot gain.xe2x80x9d When printing a single small dot, for example, a small circular area, the radius of the area will increase by some small distance. Therefore, if one attempts to reproduce a halftone region of a certain coverage percentage, the effect will be to increase the perceived coverage percentage. This causes a shift in the grey scale, and in the case of color images, a shift in the tone.
Dot gain compensation is a technique commonly used to compensate for the dot gain by reducing the input area coverage prior to printing. This typically is done electronically using a lookup table that maps desired dot percentage to what dot percentage to apply in order for the desired dot percentage to be achieved. The lookup table may be generated by printing step wedges of desired coverage percentages and measuring the resulting coverage percentages after imagesetting and printing.
While traditional halftoning, the most common form of halftoning, uses small dots, with either the size of the dots or the frequency of dots per unit area varying to provide for different grey tones, other forms of halftoning also are known that use small graphic elements of varying size or frequency that are not dot-like. Non-traditional halftoning schemes include:
halftoning using thin lines segments that have different thicknesses. Such techniques are common in the design of security documents such as banknotes. See, for example, the image of George Washington in the common United States one dollar bill;
halftoning using character fonts; and
Halftoning using any other small elements arranged so that the average ink coverage per unit area is modulated according the grey scale.
Lookup table coverage gain compensation techniques such as commonly used for dot gain compensation are not very effective for compensating for the coverage gain in most non-traditional halftoning techniques.
Thus there is a need in the art for applying coverage gain compensation to the graphic elements used for non-traditional halftoning to account for the coverage gain that occurs on such elements in the reproduction process.
Coverage gain effects are known also to cause problems when printing a barcode comprising lines of varying width. Barcodes are used, for example, to print universal product codes (UPCs). Thus, artwork, for packaging for example, may comprise a is barcode. Coverage gain effects may cause bars that are too closely spaced to merge, leading to error in reading the barcode. The xe2x80x9cFilm Master Methodxe2x80x9d of verification of barcodes is known which includes exposing a film master positive or negative image of a test symbol having several lines of varying widths with smaller and smaller gaps between the lines, followed by precise measurement of the film master positive or negative image using light transmitted through the film. The xe2x80x9cbar-width-reductionxe2x80x9d parameter is determined as half the width of the gap that first gets filled up. Barcode verification systems that use film master methods are made, for example, by RJS, Inc., a subsidiary of Printronix, Inc., Santa Fe Springs, Calif. When a bar code element is included in artwork using a graphic design program, such as packaging application oriented graphic design programs, the user typically specifies the barcode, the scale of the barcode, and the bar-width-reduction parameter. The design system simply inserts the barcode element with the bar widths of the lines in the barcode reduced by twice the value of this parameter, leaving the length unaffected.
Another application where coverage gain effects are important is the printing of security documents. Such security documents may be made up of many line segments and may contain areas that are halftoned using non-traditional techniques. The reproduction of such documents is required to be highly accurate even in non-halftone areas. Such line elements typically may be thin, and so may be noticeably affected by coverage gain. For this reason, in the past, identical denomination currency bills were printed on the same or very a similar press, and manual adjustments were made to ensure minimum variability. However, it is now desired to print artwork supplied in digital form on different presses, on different batches of the same paper using different batches of the same inks. For example, it is desired to be able to print acceptable Euros (the new currency of Europe) in different countries on different presses. The different countries, for example, may customize one side of each banknote. Each press might produce a different amount of coverage area gain, which would result in images that don""t have the same appearance. It is desired to correct for the coverage area gains to make all such images of the same artwork reproduced on different presses appear the same, such appearance matching the expected appearance in print.
Coverage gain effects can be measured experimentally. Such experiments have shown that for large enough areas, larger than about 100 xcexcm in extent for high quality reproduction on high quality paper, coverage gain can be modeled by the perimeter of the area moving a fixed linear distance away. We call this distance the xe2x80x9csmear distancexe2x80x9d or xe2x80x9cspread distance.xe2x80x9d For example, reproducing a rectangle of A xcexcm by B xcexcm produces a rectangle of (A+xcex94x) xcexcm by (B+xcex94x) xcexcm, and reproducing a circular area of radius r xcexcm produces a circular ink area of radius (r+xcex94x) xcexcm, where xcex94x is the smear distance, in xcexcm. Typical smear distances are from 1 xcexcm to 200 xcexcm. In practice, smear distances may vary by as much as 50 xcexcm from printing press to printing press of similar type for the same paper and ink. Different types of printing presses may lead to variations between presses as large as 200 xcexcm. For high resolution work such as encountered in security printing, a smear distance as small as 1 xcexcm may be significant. Electronically compensating for such effects requires an imagesetter of extremely high resolution, and such imagesetters are only now coming to market. For example, the Barco Graphics SECUSETTER(trademark) and SILVERWRITER(trademark) imagesetters (Barco Graphics, Gent, Belgium, the assignee of the present invention), are capable of resolution of up to 20,000 lines per inch (lpi), and can thus be adjusted by 1.25 xcexcm.
While the coverage area gain effects typically produce larger areas, it is also possible that such effects produce a negative gain, that is, an area that is smaller in reproduction than desired. This may occur, for example, using negative plates. It also may occur when in the artwork, small white areas are on top of a dark printed background. Thus the white areas are made smaller by the spread of the darker background, leading to a negative coverage gain in the white areas. It therefore is to be understood that coverage area gain may be positive or negative.
There thus is a need for a method that compensates for coverage gain effects by automatically enlarging or reducing the area around graphic elements that would be reproduced without such compensation. So enlarging an area is called spreading or applying a spread, and so reducing the area is called choking or applying a choke.
While a coverage gain compensation technique called dot gain compensation is known for traditionally halftoned areas included in artwork, and a simple line-width compensation technique is known for compensating a barcode element included in artwork, there still is a need in the art for coverage gain compensation techniques that can compensate for coverage area gain effects for all types elements in the artwork, including small linework elements such as small elements used for non-traditional halftoning. There also is a need for a method that can be implemented electronically for compensating for coverage area effects by selecting elements that are candidates for coverage gain effects, and automatically applying a choke or spread by an appropriate amount to the elements. There also is a need for a method that can compensate for coverage area effects that are in the range of 1 to 200 xcexcm.
When reproducing very small segments, those that have a local thickness, e.g., less than about 50 xcexcm, on a high quality, high resolution press on high quality paper, the constant smear distance model is no longer accurate; and the smear distance becomes dependent on the local thickness of the segment.
Thus there is not only a need for an apparatus and method that can compensate for coverage area gain effects that are modeled by a constant spread distance, but also for coverage area gain effects that are dependent on the local thickness or shape.
It is a broad object of the invention to provide a method to compensate for coverage gain effects that occur during the reproduction of linework having solid ink coverage.
A feature of the method of the invention is that it can be implemented electronically to automatically compensate for coverage gain effects.
Another feature of the method of the invention is that it can compensate for coverage gain effects that are in the range of 1 to 200 xcexcm.
Another feature of an embodiment of the invention is that it uses the imaging ability of very high resolution imagesetters.
Another feature of the invention is a method able to compensate for coverage gain that occurs on graphic elements used for non-traditional halftoning in the reproduction process, thereby providing a method of compensating for coverage gain regions of artwork other than regions halftoned by traditional means.
A feature of one embodiment of the invention is providing a method that can compensate for coverage area gain effects that are modeled by a constant spread distance.
A feature of another embodiment of the invention is providing a method that can compensate for coverage area gain effects that are dependent on the local thickness or curvature of an element.
To summarize, one form of the invention is a method of compensating artwork for coverage gain that occurs during reproduction of the artwork on a reproduction system, the artwork including one or more graphic elements, the method comprising: characterizing the reproduction system; and modifying the artwork by causing a spread or choke to one or more selected graphic elements of artwork by an amount determined according to the results of the characterization step. By causing a spread and choke is meant that a spread and choke will eventually be applied to the element somewhere in the design and reproduction chain. Thus one way of causing a spread and choke is to mark an element for later modification of its perimeter, and another is to directly modify the element""s perimeter. Reproducing the modified artwork on the reproduction system thus reduces the coverage gain effects.
In one embodiment, the selected graphic elements are linework elements, such linework elements being, for example, the elements of a non-traditional halftoning technique.
In one implementation, the artwork comprises pixel data, and the step of modifying further comprises: identifying regions of constant ink value in the pixel data, and causing a spread or choke on the graphic elements defined by the regions, where in this case, the causing of a spread or choke is by directly changing the area of the regions.
In another implementation, the artwork is provided in an object-oriented graphic format, and reproducing the artwork includes RIPping the artwork. In such an implementation, the causing of the spread or choke of an element includes marking the element for enlargement or contraction during RIPping. Alternatively, causing the spread or choke of an element may includes directly enlarging or contracting the boundaries of the element. In yet another version, causing the spread or choke of an element comprises adding a stroke attribute to the contour of an element not having the stroke attribute, or, for an element that has a contour with a stroke attribute, modifying the stroke attribute of the contour of the element.
Typically, the choke or spread is by a linear amount in the range of 1 to 200 xcexcm throughout the perimeter. More specifically, the choke or spread is by an amount in the range of 1 to 50 xcexcm.
In the preferred embodiment, the reproduction system includes a very high resolution imagesetter.
In one version, the characterizing step determines a fixed smear distance for each ink of the reproduction system, and the modifying step includes selecting as the selected elements all linework elements of constant ink value, and causing the spread or choke by the fixed smear distance for each particular ink and for each selected linework element of the particular ink.
In another version, the characterizing step determines a relationship between the smear distance for each ink of the reproduction system and the local thickness of a line element of constant ink value, and the modifying step determines, for each particular ink and for each selected linework element of the particular ink, the local thickness of the linework element at each location of the linework element. The causing of the spread or choke is by the smear distance for the particular ink for the determined local thickness.
Another aspect of the invention is providing a design system for preparing artwork for reproduction on a reproduction system. The design system includes a computer, a graphic design program for producing artwork for reproduction from input data including a set of one or more graphic elements, and a coverage gain compensation program connected to the graphic design program and configured to cause a spread or choke to one or more selected graphic elements of the set of graphic elements thereby compensating the coverage gain effects of reproducing the selected elements.
Other features and properties will be clear from the detailed description that follows.